Gravimeters are used in geological exploration to measure the first derivatives of the earth's gravitational field. Whilst some advances have been made in developing gravimeters which can measure the first derivatives of the earth's gravitational field because of the difficulty in distinguishing spatial variations of the field from temporal fluctuations of accelerations of a moving vehicle, these measurements can usually be made to sufficient precision for useful exploration only with land-based stationary instruments.
Gravity gradiometers (as distinct from gravimeters) are used to measure the second derivative of the gravitational field and use a sensor which is required to measure the differences between gravitational forces down to one part in 1012 of normal gravity.
Typically such devices have been used to attempt to locate deposits such as ore deposits including iron ore and geological structures bearing hydrocarbons.
The gravity gradiometer typically has at least one sensor in the form of sensor mass which is pivotally mounted for movement in response to the gravity gradient.
International publication WO 90/07131, partly owned by the present applicant's associated company, discloses such a gravity gradiometer. Gravity gradiometers of that type are typically mounted in an aircraft and carried by the aircraft while making measurements. The consequence of this is that the gravity gradiometer can move with movements of the aeroplane. This creates accelerations of the gradiometer which are detected by the gravity gradiometer and if not compensated for, will produce noise or swamp actual accelerations or movement of the gradiometer in response to the gravity gradient which is to be detected by the gravity gradiometer.
The gravity gradiometer disclosed in International publication WO 90/07131 includes two sensor masses which are orthogonally positioned and arranged to move about a common axis. The sensor masses are suspended by pivots and can oscillate in planes that are orthogonal to the common axis. For measurement of the gravity gradient the instrument is continuously rotated and a local change in the gravity gradient results in oscillating of both sensor masses relative to a rotated housing of the instrument. Such arrangement has the advantage that at least some unwanted accelerations, such as those resulting from a sudden movement of an aircraft, are experienced by both sensor masses in the same manner and can be eliminated.
The forces that result in such oscillation are very small and for proper operation of the gravity gradiometer each sensor mass should be balanced so that each sensor mass has the same dynamic properties, which provides a technological challenge.
The gravity gradiometer typically is moved relatively fast in an aircraft over a ground plane. As described above, the instrument with sensor masses is continuously rotated and a change in gravitational gradient causes the oscillating movement of the sensor masses relative to a housing. Typically the angular frequency of the rotation is chosen so that the sensor masses oscillate at or near resonance frequency, which increases sensitivity. Both sensor masses should have the same resonance frequency and the same mass.
Further, the bandwidth associated with the resonant oscillation of the sensor masses should be relatively large as the bandwidth determines the spatial resolution with which changes in the gravitational gradient can be detected when the apparatus is flown over a ground plane.
The present invention provided technological advancement.